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Materials of the Deep

April 13, 2015

The open sea has always had an air of mystery. Just as our ancestors imagined strange underworlds beneath the surface of Earth, they imagined strange things existing in the great ocean depths where they couldn't venture. That's why older maps had images of strange sea creatures and captions such as, "Here, there be monsters."

Sea monster detail from the Carta Marina by Olaus Magnus, 1539Here, there be monsters!

Detail from the Carta Marina by Olaus Magnus, 1539. The Carta Marina was a "Marine map and Description of the Northern Lands and of their Marvels."

(Via Wikimedia Commons.)

Despite the perceived dangers, men were still willing to set out to sea to harvest its wealth of food stocks, and also its natural material resources. One of the chemicals harvested from the sea in antiquity was Tyrian purple, also known as royal purple. This extract from the sea snail, Bolinus brandaris, is a natural dye. It was expensive to produce; thus, the "royal" appellation.

Some marine materials are especially easy to harvest since you can find them on land. Limestone, a very useful building material, is formed from the the skeletal remains of marine organisms, such as Foraminifera. The metamorphic form of limestone, marble, was an important structural material in antiquity. The skeletal remains are chemically calcium carbonate (CaCO3), but in two different crystalline forms, calcite and aragonite.

Iceland spar (calcite) birefringence
Optical physics would not have advanced as quickly as it did without calcite. The birefringence of calcite allowed construction of early polarizers. (Left image, a photograph of a crystal of Iceland spar calcite by Parent Géry; right image, schematic diagram of birefringence(modified) by Mikael Häggström, via Wikimedia Commons.)

The question naturally arises as to why nature would have its marine creatures produce two different forms of calcite as a skeletal material, especially since the aragonite form is a metastable form that's more soluble, especially in acidic waters. As a team of scientists from the Massachusetts Institute of Technology (Cambridge, Massachusetts) and Lawrence Berkeley National Laboratory (Berkeley, California) write in a recent issue of the Proceedings of the National Academy of Science, the choice of crystal habit is actually an "accident of birth."[1-2]

"Birth" in the case of crystal growth is the nucleation phase when the first few atoms of material arrange themselves into the template on which subsequent crystal will grow. As the scientists discovered, the magnesium concentration in the water is the determining factor. The calcium-to-magnesium ratio affects the surface energy of the nucleating crystals, and beyond a certain ratio, aragonite is favored over calcite.[1-2]

Calcite is a trigonal crystal with unit cell dimensions, a = 4.99 pm, and c = 1706 pm, while aragonite is an orthorhombic crystal with unit cell dimensions, a = 495 pm, b = 796 pm, and c = 574 pm. It's easy to see why magnesium substituting for calcium would have a large affect on crystal energy. The ionic radius of Ca2+ is 114 pm, while the ionic radius of Mg2+ is 86 pm, so substitution of magnesium for calcium would lead to considerable lattice strain.

A calcite and aragonite crystal (MIT image)A calcite crystal (upper left) with an aragonite crystal.

(MIT photograph by Felice Frankel, licensed under a Creative Commons Attribution Non-Commercial No Derivatives license.)

The metastable forms of elements and compounds generally have different properties than the stable form. One example is diamond, a metastable form of carbon with extreme hardness. In the case of calcium carbonate, the aragonite phase is more soluble, which has implications for the natural sequestration of carbon dioxide in seawater.[1-2] The aragonite shells of marine life are more vulnerable to ocean acidification, which is an effect of climate change.[1]

The experimental measurement of surface energy is difficult, so the research team turned to atomic-level calculations. The calculations revealed that magnesium concentration causes the surface energy of calcite to increase to a point at which nucleation is diminished by orders of magnitude for that crystal phase. While calcite growth is impeded, the metastable aragonite phase is favored.[2] This calculation is general enough that it could be applied to the crystallization of other compounds from solution.[2]

A scientific understanding of how metastable phases are created from solutions would be beneficial. Faster dissolving pharmaceuticals would be useful, as would more stable photocatalysts. The research team is developing their atomic model into a method to predict other material properties, such as chemical reactivity, electrical conductivity, and hardness. This research was funded by the U.S. Department of Energy and the National Science Foundation.[2]

Another research study on a material from the sea, limpet teeth, has just been published as an open access paper in the Journal of the Royal Society Interface.[3] In this study, it was found that limpet teeth are formed from the strongest natural material found to date, stronger than spider silk and stronger than many man-made materials.[3-7] I wrote about spider silk in two previous articles, Spider Silk, March 12, 2012 and Spider Silk Mechanics, February 15, 2013.

Ventral and  dorsal views of limpets.Ventral (upper left) and dorsal views of limpets.

(Photo by Janek Pfeifer, via Wikimedia Commons.)

Limpet teeth are actually part of the limpet tongue, called a radula, and they are used to scrape food from rock surface. For such a purpose, these teeth need to be strong, so strong that they actually scrape away some rock, which is swallowed and is excreted as hardened blocks in their fecal pellets.[4] Limpet teeth are made from a composite of protein material and the mineral, goethite (α-FeO(OH)) that exists as needle-like crystals. The composite is structured much like carbon fiber reinforced plastic.[4]

Since limpet teeth are very small, of the order of 100 μm, mechanical measurement of their strength is difficult. The research team used in situ atomic force microscopy for tensile strength measurements, after first milling teeth to the typical "dog bone" specimen shape in which the central area was a hundred times thinner than a human hair.[3-4] The specimens were pulled inside the atomic force microscope until they broke.[4]

Stress-strain curves for several limpet tooth specimensStress-strain curves for several limpet tooth specimens of different lengths.

(Fig. 5 of ref. 3, licensed under a Creative Commons Attribution License .)[3]

Although the measurement is difficult, the interpretation of the data is straightforward. The tensile strength that they found was in the range 3.0 to 6.5 GPa, it was independent of sample size, and about five time greater than the tensile strength of spider silk.[3-5] The high strength is attributed to the nanoscale dimension of the goethite reinforcing fibers. These fibers have a diameter less than the critical flaw size as developed in Griffith's fracture theory of glass, and this suggests that the material was optimized by natural selection for high strength.[3]

Says the lead author of the limpet study, Asa Barber, a professor at the University of Portsmouth (Portsmouth, UK),
Biology is a great source of inspiration when designing new structures but with so many biological structures to consider, it can take time to discover which may be useful.”[6]

References:

  1. Wenhao Sun, Saivenkataraman Jayaraman, Wei Chen, Kristin A. Persson, and Gerbrand Ceder, "Nucleation of metastable aragonite CaCO3 in seawater," Proc. Natl. Acad. Sci. (published ahead of print, March 4, 2015), doi:10.1073/pnas.1423898112.
  2. David L. Chandler, "Mystery solved: Why seashells’ mineral forms differently in seawater," MIT Press Release, March 2, 2015.
  3. Asa H. Barber , Dun Lu , and Nicola M. Pugno, "Extreme strength observed in limpet teeth," J. R. Soc. Interface, vol. 12 (February 15, 2015), article no. 20141326, DOI: 10.1098/rsif.2014.1326. This is an open access publication with a PDF file available here.
  4. Jonathan Webb, "Limpet teeth set new strength record," BBC News, February 18, 2015.
  5. Strongest material known to man? A limpet's tooth, Telegraph (UK), February 18, 2015.
  6. Scientists find strongest natural material, University of Portsmouth Press Release, February 18, 2015.
  7. Limpets' teeth consist of the strongest biological material, scientists say, BBC, February 18, 2015.

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